A prior art window structure semiconductor laser is described in "An AlGaAs Window Structure Laser", IEEE Journal of Quantum Electronics, Vol. QE-15, No. 8, August, 1979, pp. 775-781. FIG. 3(a) is a plan view of such a device, while FIGS. 3(b) and 3(c) are cross-sectional views taken along lines 3b--3b and 3c--3c of FIG. 3(a), respectively.
As illustrated in FIGS. 3(a)-3(c), a semiconductor light emitting device using the growth techniques of the prior art starts with an n-type GaAs substrate 1. A first cladding layer 2, of n-type Al.sub.y Ga.sub.1-y As is grown on the n-type GaAs substrate 1. Thereafter, an active layer 3 is grown on the first cladding layer 2 through conventional growth techniques. As is understood in the art, active layer 3 is either of n-type or undoped Al.sub.x Ga.sub.1-x As(x&lt;y). Similar to the first cladding layer 2, a second cladding layer 4 of the same n-type Al.sub.y Ga.sub.1-y As the first cladding layer 2 is next grown on the active layer 2. The cladding layers 2, 4 have a lower refractive index than the active layer 2, thus confining light generated in the active layer to within that layer. Contact layer 5 is then grown on cladding layer 4 and is of n-type GaAs, completing the growth process of a prior art semiconductor light emitting device. Although not illustrated in FIG. 3, an n side electrode is formed on the substrate 1 and a p side electrode on the contact layer 5 for making electrical connections with the semiconductor device.
In order to create a p-n junction for injecting carriers into a predetermined section of the active layer 3, a first p-type impurity region 6 is diffused from the surface of the contact layer 5 through the second cladding layer 4 to approximately the junction between the second cladding layer 4 and the active layer 3. Thereafter, thermal processing produces a second p-type diffusion region 7 by out-diffusing impurities from the first p-type diffusion region 6 into the active layer 3 to approximately the boundary between the active layer 3 and first cladding layer 2. Thus, there is created a stripe region 8 in the active layer which (a) forms a p-n junction with the adjacent portion of the first cladding layer, and (b) creates a resonator which has a higher index of refraction than the cladding layers or the undoped or n-doped regions of the active layer to confine light within the resonator.
As best shown in FIGS. 3(a) and 3(c), the first and second diffusion regions 6, 7 are positioned longitudinally of the laser, but terminate short of the facets 10, 10' which are located on either longitudinal end surface of the laser. Thus, a pair of windows 9, 9' are created between the stripe region or resonator 8 and the facets 10, 10'. By virtue of the impurity concentration in the resonator 8 (which it is recalled is created by out-diffusion from the stripe region 7), the energy band gap of the resonator 8 is smaller than that of the undoped or n-type doped region of the active layer 3 at the windows 9, 9'. By virtue of this difference in energy gaps, very little of the light generated in the resonator 8 is absorbed in the windows 9, 9', allowing the laser to be operated at higher power.
Although this prior art light emitting device solves the problem of device failure at high operating power by providing windows interposed between the resonator and the laser facets, in contrast to other prior devices where the resonator intersects the facets, it appears to suffer from the problem of inadequate yield because of manufacturing difficulties. More particularly, the location of the out-diffusion region 7, particularly the portion which forms the resonator 8 must be carefully controlled in order to produce useful laser devices. Preferably, the resonator portion 8 of the out-diffusion region 7 penetrates the active layer 3 and terminates at the boundary between the active layer and first cladding layer 2. However, because of the difficulty of controlling the diffusion depth of the first diffusion region 6, and consequently the difficulty of controlling the location of the significantly smaller out-diffusion region 7, it often happens that the resonator portion 8 of the out-diffusion region 7 does not reach the active layer, or alternatively it penetrates the active layer and protrudes into the first cladding layer 2. Both of these situations produce laser devices which are non-functional, thus reducing the yield of the process. When it is appreciated that the cladding layers are on the order of 2 or more microns whereas the active layer is typically about 0.1 microns, the difficulty of process control will be apparent, first of all, in controlling the depth of the first diffusion region to provide an appropriate starting point for the out-diffusion, and subsequently in controlling the out-diffusion to form the resonator. This problem can be further aggravated by surface irregularities on the grown crystal which can be exacerbated by the multiple levels which are grown before the first diffusion region is formed.